Quantification in emission tomography: Challenges, solutions, and performance

Emission tomography (single photon emission tomography—SPECT—and positron emission tomography—PET) offers a great potential for the quantitative characterization of functional and molecular processes in vivo. Indeed, voxel values in the reconstructed SPECT and PET images can theoretically be translated into a well-understood physical quantity, namely radiotracer concentration, from which all kinds of parameters characterizing molecular processes can be derived using appropriate modeling. Such quantitative interpretation of SPECT and PET images is often referred to as “quantification”. The objective of this short review is to examine how far we are in this quest for quantification by presenting the different problems that have to be addressed, the methods that have been developed to solve these problems, and the current performance of these methods.